The present invention relates generally to fuel vapor purge control for automotive vehicles equipped with computer control evaporative emission systems and more particularly to an electronically controlled variable area purge regulator.
Virtually all new automotive vehicles manufactured in the United States are equipped with emission control systems that are operable for limiting the emission of hydrocarbons into the atmosphere. One aspect of these emission control systems typically involves an evaporative emission control system which traps fuel vapors emitted from the fuel tank in a carbon-filled canister. The evaporative emission control system is periodically purged by drawing the fuel vapors from the canister into the engine intake system. In this manner, fuel vapors from the fuel tank are delivered to the engine for subsequent combustion.
Conventional evaporative emission control systems are equipped with a fixed area purge valve for regulating the flow rate of fuel vapors introduced into the intake system in response to the pressure difference between the intake manifold and atmosphere. These purge valves utilize a pulse width modulated (PWM) solenoid valve which is responsive to a duty cycle control signal from an engine controller unit (ECU) for selectively establishing and terminating communication between the canister and the intake system. However, these purge valves provide uneven flow characteristics, particularly at low engine speeds, and also do not provide consistent flow control independent of variations in manifold vacuum and inlet pressure.
More recent developments in this area include a vapor management valve which uses a diaphragm vacuum regulator in combination with an electric vacuum regulator (EVR) valve that regulates a vacuum signal in accordance with the current signal supplied thereto by the ECU. In operation, the vapor management valve is able to generate substantially linear output flow characteristics between two calibration points as a function of the solenoid current in a manner that is independent of changes in manifold vacuum of the engine intake system. U.S. Pat. No. 5,277,167, which is commonly owned by the assignee of the present invention and expressly incorporated by reference herein, discloses a vapor management valve which represents a significant improvement over the conventional PWM solenoid valves.
While the vapor management valve provides an advancement over a wide range of operating conditions, in this technology continuous improvements have been sought particularly with respect to extreme operating conditions. More specifically, the vapor management valve, which is referenced to atmospheric pressure, is designed to withstand relatively high canister pressures without leaking when the engine is off. However, the vacuum generated by the EVR may bias the diaphragm to a near open condition such that relatively low canister pressures (cracking pressure) can possibly open the valve and cause uncommanded purge flow. The vapor management valve also requires a continuous flow of air through the EVR into the intake manifold to operate. Accordingly, certain engine operating conditions, particularly in low friction engines having several devices that operate on engine vacuum, can result in a cumulative bleed flow which can exceed the desired idle air flow requirements of the engine resulting in excessive and/or fluctuating engine RPM. Likewise, the flow of air into the EVR can be restricted when the intake air filter of the vapor management valve becomes clogged, for example with snow, dust or dirt, or alternately when water is ingested therethrough. If the flow of air is sufficiently restricted, the vapor management valve will not perform as desired.
In view of increasingly stringent emission regulations, the demands on evaporative emission control systems have increased dramatically. In particular, in order to satisfy current Environmental Protection Agency (EPA) emission requirements, the purge flow through the canister must be increased. To achieve this result within the EPA city test cycle, it is therefore necessary to provide purge flow at engine idle speeds. Moreover, purge flow control must also be accurately regulated across the entire engine operating range so as not to cause unacceptable exhaust emissions.
To provide such enhanced flow control, it is desirable to have the output flow characteristics of the purge valve be continuous and proportional to the duty cycle of the electric control signal applied to the valve, even at low engine speeds, and yet be independent of variations to the inlet pressure and outlet manifold vacuum applied across the valve. Accordingly, the output flow of the valve should be substantially continuous at a given duty cycle control signal and be controllable in response to regulated changes in the duty cycle regardless of these pressure variations. Moreover, it is also desirable that the output flow of the purge regulator vary nonlinearly over the duty cycle range.
While the above-described flow regulators have been generally successful in providing substantially linear output flow between a given range of duty cycle, they have been unable to achieve the desirable non-linear output flow characteristics of the above-noted performance specifications. Accordingly, there is a continuing need to develop alternatives which meet these performance specifications and which can be manufactured and calibrated in a more efficient and cost effective manner.